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\answer $1+\Bigl[\dfrac{R(t)}{R_{0}}\Bigr]^{3}-2\Bigl[\dfrac{R(t)}{R_{0}}\Bigr]^{\frac{3}{2}}=6 \pi G \rho_{0}\left(t-t_{0}\right)^{2}$ \\
式中$ t _ { 0 } $表示现在的时刻，$ \rho _ 0 $及$ R _ 0 $分别是$ t _ { 0 } $时刻的平均密度及宇宙尺
度因子\vspace{-0.25em}

\answer 因为
$\dfrac{\dif^{2} R(t)}{\dif t^{2}}=-\dfrac{4 \pi G}{3}\cdot\dfrac{\rho_{0} R_{0}^{3}}{R_0}<0,\dfrac{\dif}{\dif t}\Bigl[\dfrac{\dif R(t)}{\dif t}\Bigr]<0$\\
所以是减速的
